1. Field of the Invention
The present invention concerns novel (co)polymers and a novel radical polymerization process based on transition metal-mediated atom or group transfer polymerization (“atom transfer radical polymerization”).
2. Discussion of the Background
Living polymerization renders unique possibilities of preparing a multitude of polymers which are well-defined in terms of molecular dimension, polydispersity, topology, composition, functionalization and microstructure. Many living systems based on anionic, cationic and several other types of initiators have been developed over the past 40 years (see O. W. Webster, Science, 251, 887 (1991)).
However, in comparison to other living systems, living radical polymerization represented a poorly answered challenge prior to the present invention. It was difficult to control the molecular weight and the polydispersity to achieve a highly uniform product of desired structure by prior radical polymerization processes.
On the other hand, radical polymerization offers the advantages of being applicable to polymerization of a wide variety of commercially important monomers, many of which cannot be polymerized by other polymerization processes. Moreover, it is easier to make random copolymers by radical polymerization than by other (e.g., ionic) polymerization processes. Certain block copolymers cannot be made by other polymerization processes. Further, radical polymerization processes can be conducted in bulk, in solution, in suspension or in an emulsion, in contrast to other polymerization processes.
Thus, a need is strongly felt for a radical polymerization process which provides (co)polymers having a predetermined molecular weight, a narrow molecular weight distribution (low “polydispersity”), various topologies and controlled, uniform structures.
Three approaches to preparation of controlled polymers in a “living” radical process have been described (Greszta et al, Macromolecules, 27, 638 (1994)). The first approach involves the situation where growing radicals react reversibly with scavenging radicals to form covalent species. The second approach involves the situation where growing radicals react reversibly with covalent species to produce persistent radicals. The third approach involves the situation where growing radicals participate in a degenerative transfer reaction which regenerates the same type of radicals.
There are some patents and articles on living/controlled radical polymerization. Some of the best-controlled polymers obtained by “living” radical polymerization are prepared with preformed alkoxyamines or are those prepared in situ (U.S. Pat. No. 4,581,429; Hawker, J. Am. Chem. Soc., 116, 11185 (1994); Georges et al, WO 94/11412; Georges et al, Macromolecules, 26, 2987 (1993)). A Co-containing complex has been used to prepare “living” polyacrylates (Wayland, B. B., Pszmik, G., Mukerjee, S. L., Fryd, M. J. Am. Chem. Soc., 116, 7943 (1994)). A “living” poly(vinyl acetate) can be prepared using an Al(i-Bu)3: Bpy:TEMPO initiating system (Mardare et al, Macromolecules, 27, 645 (1994)). An initiating system based on benzoyl peroxide and chromium acetate has been used to conduct the controlled radical polymerization of methyl methacrylate and vinyl acetate (Lee et al, J. Chem. Soc. Trans. Faraday Soc. I, 74, 1726 (1978); Mardare et al, Polym. Prep. (ACS), 36(1) (1995)).
However, none of these “living” polymerization systems include an atom transfer process based on a redox reaction with a transition metal compound.
One paper describes a redox iniferter system based on Ni(O) and benzyl halides. However, a very broad and bimodal molecular weight distribution was obtained, and the initiator efficiency based on benzyl halides used was about 1-2% or less (T. Otsu, T. Tashinori, M. Yoshioka, Chem. Express 1990, 5(10), 801). Tazaki et al (Mem. Fac. Eng., Osaka City Univ., vol. 30 (1989), pages 103-113) disclose a redox iniferter system based on reduced nickel and benzyl halides or xylylene dihalides. The examples earlier disclosed by Tazaki et al do not include a coordinating ligand. Tazaki et al also disclose the polymerization of styrene and methyl methacrylate using their iniferter system.
These systems are similar to the redox initiators developed early (Bamford, in Comprehensive Polymer Science, Allen, G., Aggarwal, S. L., Russo, S., eds., Pergamon: Oxford, 1991, vol. 3, p. 123), in which the small amount of initiating radicals were generated by redox reaction between (1) RCHX2 or RCX3 (where X═Br, Cl) and (2) Ni(0) and other transition metals. The reversible deactivation of initiating radicals by oxidized Ni is very slow in comparison with propagation, resulting in very low initiator efficiency and a very broad and bimodal molecular weight distribution.
Bamford (supra) also discloses a Ni[P(OPh)3]4/CCl4 or CBr4 system for polymerizing methyl methacrylate or styrene, and use of Mo(CO)n to prepare a graft copolymer from a polymer having a brominated backbone and as a suitable transition metal catalyst for CCl4, CBr4 or CCl3CO2Et initiators for polymerizing methyl methacrylate. Organic halides other than CCl4 and CBr4 are also disclosed. Mn2(CO′10/CCl4 is taught as a source of CCl3 radicals. Bamford also teaches that systems such as Mn(acac)3 and some vanadium (V) systems have been used as a source of radicals, rather than as a catalyst for transferring radicals.
A number of the systems described by Bamford are “self-inhibiting” (i.e., an intermediate in initiation interferes with radical generation). Other systems require coordination of monomer and/or photoinitiation to proceed. It is further suggested that photoinitiating systems result in formation of metal-carbon bonds. In fact, Mn(CO)5Cl, a thermal initiator, is also believed to form Mn—C bonds under certain conditions.
In each of the reactions described by Bamford, the rate of radical formation appears to be the rate-limiting step. Thus, once a growing radical chain is formed, chain growth (propagation) apparently proceeds until transfer or termination occurs.
Another paper describes the polymerization of methyl methacrylate, initiated by CCl4 in the presence of RuCl2(PPh3)3. However, the reaction does not occur without methylaluminum bis(2,6-di-tert-butylphenoxide), added as an activator (see M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura, Macromolecules, 28, 1721 (1995)).
U.S. Pat. No. 5,405,913 (to Harwood et al) discloses a redox initiating system consisting of CuII salts, enolizable aldehydes and ketones (which do not contain any halogen atoms), various combinations of coordinating agents for CuII and CuI, and a strong amine base that is not oxidized by CuII. The process of Harwood et al requires use of a strong amine base to deprotonate the enolizable initiator (thus forming an enolate ion), which then transfers a single electron to CuII, consequently forming an enolyl radical and CuI. The redox initiation process of Harwood et al is not reversible.
In each of the systems described by Tazaki et al, Otsu et al, Harwood et al and Bamford, polymers having uncontrolled molecular weights and polydispersities typical for those produced by conventional radical processes were obtained (i.e., >1.5). Only the system described by Kato et al (Macromolecules, 28, 1721 (1995)) achieves lower polydispersities. However, the polymerization system of Kato et al requires an additional activator, reportedly being inactive when using CCl4, transition metal and ligand alone.
Atom transfer radical addition, ATRA, is a known method for carbon-carbon bond formation in organic synthesis. (For reviews of atom transfer methods in organic synthesis, see Curran, D. P. Synthesis, 1988, 489; Curran, D. P. in Free Radicals in Synthesis and Biology, Minisci, F., ed., Kluwer: Dordrecht, 1989, p. 37; and Curran, D. P. in Comprehensive Organic Synthesis, Trost, B. M., Fleming, I., eds., Pergamon: Oxford, 1991, Vol. 4, p. 715.) In a very broad class of ATRA, two types of atom transfer methods have been largely developed. One of them is known as atom abstraction or homolytic substitution (see Curran et al, J. Org. Chem., 1989, 54, 3140; and Curran et al, J. Am. Chem. Soc., 1994, 116, 4279), in which a univalent atom (typically a halogen) or a group (such as SPh or SePh) is transferred from a neutral molecule to a radical to form a new σ-bond and a new radical in accordance with Scheme 1 below: Scheme 1: 
In this respect, iodine atom and the SePh group were found to work very well, due to the presence of very weak C—I and C—SePh bonds towards the reactive radicals (Curran et al, J. Org. Chem. and J. Am. Chem. Soc., supra). In earlier work, the present inventors have discovered that alkyl iodides may induce the degenerative transfer process in radical polymerization, leading to a controlled radical polymerization of several alkenes. This is consistent with the fact that alkyl iodides are outstanding iodine atom donors that can undergo a fast and reversible transfer in an initiation step and degenerative transfer in a propagation step (see Gaynor et al, Polym. Prep. (Am. Chem. Soc., Polym. Chem. Div.), 1995, 36(1), 467; Wang et al, Polym. Prep. (Am. Chem. Soc., Polym. Chem. Div.), 1995, 36(1), 465; Matyjaszewski et al, Macromolecules, 1995, 28, 2093). By contrast, alkyl bromides and chlorides are relatively inefficient degenerative transfer reagents.
Another atom transfer method is promoted by a transition metal species (see Bellus, D. Pure & Appl. Chem. 1985, 57, 1827; Nagashima, H.; Ozaki, N.; Ishii, M.; Seki, K.; Washiyama, M.; Itoh, K. J. Org. Chem. 1993, 58, 464; Udding, J. H.; Tuijp, K. J. M.; van Zanden, M. N. A.; Hiemstra, H.; Speckamp, W. N. J. Org. Chem. 1994, 59, 1993; Seijas et al, Tetrahedron, 1992, 48(9), 1637; Nagashima, H.; Wakamatsu, H.; Ozaki, N.; Ishii, T.; Watanabe, M.; Tajima, T.; Itoh, K. J. Org. Chem. 1992, 57, 1682; Hayes, T. K.; Villani, R.; Weinreb, S. M. J. Am. Chem. Soc. 1988, 110, 5533; Hirao et al, Syn. Lett., 1990, 217; and Hirao et al, J. Synth. Org. Chem. (Japan), 1994, 52(3), 197; Iqbal, J; Bhatia, B.; Nayyar, N. K. Chem. Rev., 94, 519 (1994); Asscher, N., Vofsi, D. J. Chem. Soc. 1963, 1887; and van de Kuil et al, Chem. Mater., 1994, 6, 1675). In these reactions, a catalytic amount of transition metal compound acts as a carrier of the halogen atom in a redox process.
Initially, the transition metal species, Mtn, abstracts halogen atom X from the organic halide, R—X, to form the oxidized species, Mtn+1X, and the carbon-centered radical R−. In the subsequent step, the radical, R−, reacts with alkene, M, with the formation of the intermediate radical species, R—M−. The reaction between Mtn+1X and R—M− results in the target product, R—M—X, and regenerates the reduced transition metal species, Mtn, which further reacts with R—X and promotes a new redox process.
The high efficiency of transition metal-catalyzed atom transfer reactions in producing the target product, R—M—X, in good to excellent yields (often >90%) may suggest that the presence of an Mtn/Mtn+1 cycle-based redox process can effectively compete with the bimolecular termination reactions between radicals (see Curran, Synthesis, in Free Radicals in Synthesis and Biology, and in Comprehensive organic Synthesis, supra). However, the mere presence of a transition metal compound does not ensure success in telomerization or polymerization, even in the presence of initiators capable of donating a radical atom or group. For example, Asscher et al (J. Chem. Soc., supra) reported that copper chloride completely suppresses telomerization.
Furthermore, even where a transition metal compound is present and telomerization or polymerization occurs, it is difficult to control the molecular weight and the polydispersity (molecular weight distribution) of polymers produced by radical polymerization. Thus, it is often difficult to achieve a highly uniform and well-defined product. It is also often difficult to control radical polymerization processes with the degree of certainty necessary in specialized applications, such as in the preparation of end functional polymers, block copolymers, star (co)polymers, etc. Further, although several initiating systems have been reported for “living”/controlled polymerization, no general pathway or process for “living”/controlled polymerization has been discovered.
Copolymerization of electron-donor type monomers (unsaturated hydrocarbons, vinyl ethers, etc.) with electron acceptor type monomers (acrylates, methacrylates, unsaturated nitrites, unsaturated ketones, etc.) in the presence of monomer complexing agents (ZnCl2, Et3Al2Cl3, etc.) yield highly, if not strictly alternating copolymers (Hirooka et al, J. Polym. Sci. Part B, 5, 47 (1967); Furukawa et al, Rubber Chem. Technol., 51(3), 601 (1979)). The copolymerization succeeded, however, only if the polar monomer was significantly complexed by the Lewis acid. Further, the copolymerization was often initiated spontaneously, thus yielding very high molecular weight products having broad polydispersities. The mechanism of this reaction is controversial and there are suggestions that it is due to a complex (Hirai, J. Polym. Sci. Macromol. Rev., 11, 47 (1976)) or to enhanced cross-propagation rates (Bamford et al, J. Polym. Sci. Polym. Lett. Ed., 19, 229 (1981) and J. Chem. Soc. Faraday Trans. 1, 78, 2497 (1982)).
In the radical copolymerization of isobutylene (IB) and acrylic esters, the resulting copolymers contain at most 20-30% of IB and have low molecular weights because of degradative chain transfer of IB (U.S. Pat. Nos. 2,411,599 and 2,531,196; and Mashita et al, Polymer, 36, 2973 (1995).
Conjugated monomers such acrylic esters and acrylonitrile react with donor monomers such as propylene, isobutylene, styrene in the presence of alkylaluminum halide to give 1:1 alternating copolymers (Hirooka et al, J. Polym. Sci. Polym. Chem., 11, 1281 (1973)). The alternating copolymer was obtained when [Lewis acid]0/[acrylic esters]=0.9 and [IB]0>[acrylic esters]0. The copolymer of IB and methyl acrylate (MA) obtained by using ethyl aluminum sesquichloride and 2-methyl pentanoyl peroxide as an initiating system is highly alternating, with either low (Kuntz et al, J. Polym. Sci. Polym. Chem., 16, 1747 (1978)) or high (60%) isotacticity in the presence of EtAlCl2 (10 molar % relative to MA) at 50° C. (Florjanczyk et al, Makromol. Chem., 183, 1081 (1982)).
Recently, alkyl boron halide was found to have a much higher activity than alkyl aluminum halide in alternating copolymerization of IB and acrylic esters (Mashita et al, Polymer, 36, 2983 (1995)). The polymerization rate has a maximum at about −50° C. and decreased significantly above 0° C. The copolymerization is controlled by O2 in terms of both rate and molecular weight. The alternating copolymer was obtained when [IB]0>[Acrylic esters]0. Stereoregularity was considered to be nearly random. The copolymer is an elastomer of high tensile strength and high thermal decomposition temperature. The oil resistance is very good, especially at elevated temperatures, and the hydrolysis resistance was excellent compared to that of the corresponding poly(acrylic ester)s (Mashita et al, supra).
Dendrimers have recently received much attention as materials with novel physical properties (D. A. Tomalia, A. M. Naylor, W. A. G. III, Angew. Chem., Int. Ed. Engl. 29, 138 (1990); J. M. J. Frechet, Science 263, 1710 (1994)). These polymers have viscosities lower than linear analogs of similar molecular weight, and the resulting macromolecules can be highly functionalized. However, the synthesis of dendrimers is not trivial and requires multiple steps, thus generally precluding their commercial development.
Polymers consisting of hyperbranched phenylenes (O. W. Webster, Y. H. Kim, J. Am. Chem. Soc. 112, 4592 (1990) and Macromolecules 25, 5561 (1992)), aromatic esters (J. M. J. Frechet, C. J. Hawker, R. Lee, J. Am. Chem. Soc., 113, 4583 (1991)), aliphatic esters (A. Hult, E. Malmstrom, M. Johansson, J. Polym. Sci. Polym. Ed. 31, 619 (1993)), siloxanes (L. J. Mathias, T. W. Carothers, J. Am. Chem. Soc. 113, 4043 (1991)), amines (M. Suzuki, A. Li, T. Saegusa, Macromolecules 25, 7071 (1992)) and liquid crystals (V. Percec, M. Kawasumi, Macromolecules 25, 3843 (1992)) have been synthesized in the past few years.
Recently, a method has been described by which functionalized vinyl monomers could be used as monomers for the synthesis of hyperbranched polymers by a cationic polymerization (J. M. J. Frechet, et al., Science 269, 1080 (1995)). The monomer satisfies the AB2 requirements for formation of hyperbranched polymers by the vinyl group acting as the difunctional B group, and an additional alkyl halide functional group as the A group. By activation of the A group with a Lewis acid, polymerization through the double bond can occur. In this method, 3-(1-chloroethyl)-ethenylbenzene was used as a monomer and was cationically polymerized in the presence of SnCl4.
A need is strongly felt for a radical polymerization process which provides (co)polymers having a predictable molecular weight and a controlled molecular weight distribution (“polydispersity”). A further need is strongly felt for a radical polymerization process which is sufficiently flexible to provide a wide variety of products, but which can be controlled to the degree necessary to provide highly uniform products with a controlled structure (i.e., controllable topology, composition, stereoregularity, etc.), many of which are suitable for highly specialized uses (such as thermoplastic elastomers, end-functional polymers for chain-extended polyurethanes, polyesters and polyamides, dispersants for polymer blends, etc.).